One of these days, I'm going to buy a grinding jig. I attach the pad, pencil mark the outline of the butt to the hard rubber part of the pad and with the pad removed - rough shape on my 10" disc sander. Note, I use a shop vac to help keep down the rubber dust.
Next I put a layer of masking tape around the wood and screw the pad back on. I've had good finishing results using a Dremel sanding disc and varying the speed dependant upon the rubber composition. It's eye balling that determines the final pad shape conforming to the buttstock lines.
